Consider the photograph to the right of aeolian sandstone in Utah. Yet you will also recall that the cross-beds are laid down in sets formed as the dune rolls across the desert landscape, and these are much more nearly horizontal. After all, some sediment is not originally deposited in flat beds, as you will know from the articles on sedimentology: for example aeolian sand is laid down in cross-beds, and the angle of repose of a sand dune can be as much as 34º from the horizontal. The principle of original horizontality states that sediment is originally laid down flat. Once we have used these indications to discover which way up the rocks were when the sediment was deposited, we can then apply Steno's principle to sort out their relative ages. But does it necessarily follow from this that when we look at sedimentary rocks the lowest is the oldest? For tectonic processes such as folding can completely overturn a section of rock, or, just as bad, turn it on its side so that we can't tell which was originally up and which was down.įortunately, there are many clues within the rocks which allow us to discover which way up they were originally these are known as way-up structures and will be the subject of the next article. It is all very well to say that the sediment, when originally deposited, was laid down from the bottom up. So, for example, an igneous sill intruding into sedimentary rock will be younger than the rocks immediately above it. Note that this only applies to sediment it does not necessarily apply to igneous rock, because whereas sediment is deposited from above, magma oozes up from below. This is because sediment is deposited from above, because gravity operates in a downward direction, and because sediment does not readily pass through other sediment. The principle of superposition simply says that when sediments are deposited, those which are deposited first will be at the bottom, and so the lower sediments will be the older. The principle of superposition Beds of sandstone and siltstone: the lower strata were deposited first. 2 The principle of original horizontality.
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